In similar procedures such as are known from DE-OS 38 06 660, a constant problem consists of increasing the resistance to tearing of a heat-resettable collar and/or web. For this purpose, the heat-resettable collar consists of a crosslinked polymer tape, the top side of which includes a co-extruded thermoplastic polymer coating followed by another layer of a thermoplastic polymer.
In order to increase its resistance to tearing, a fabric is inserted between these two layers and/or is arranged in the top layer. In addition to the fact that the known process is expensive and costly, there is an additional problem that heat-resettable material webs including fabrics will require a higher input heat during heat shrinking in comparison with a material web without a fabric. Moreover, the known collars and/or material webs are hard to open after shrinking due to the presence of the inserted fabric.
The forming of a heat-resettable web including a polymer top coating, a heat-resistant crosslinked layer and a heat-shrinkable reset layer is also known from DE-OS 41 26 355. In this case the heat-resettable layer consists of a crosslinked, stretched plastic web divided in shrinking direction by means of dividing sections penetrating the plastic web by forming shrink bands of a specific width. This ensures high resistance to tearing of the material web in solid and even molten condition together with the heat-stable crosslinked layer.
A process for the manufacture of heat-shrinkable sleeves from crosslinked plastics by extruding a sleeve at a temperature below crosslinking temperature of the plastic is known from AT-E 2939, with the sleeve being heated to at least crosslinking temperature, achieving the required diameter by subsequent expansion and simultaneous cooling of the sleeve. The extruded sleeve is cut to length. This length essentially corresponds to the finished sleeve or a multiple of the same. The cut sleeves are then heated to crosslinking temperature of the plastic while being retaining both on their outside and inside. Furthermore, the crosslinked sleeve will be expanded while simultaneously being cooled, to the required sleeve diameter around two or several rotating rollers arranged in parallel to each other, which are revolving away from each other. This is to eliminate problems in shape stability and uneven expansion of the sleeve.
Finally, a process for the production of crosslinked shrink tubes is known from DE-OS 27 19 308 in which the tube material is chemically crosslinked within the inflating zone in which the tube diameter is still limited to a rather small value, with a crosslinking agent blended into the plastic, such as peroxide, being caused to react by application of heat.